Power source device for electrical discharge machining

ABSTRACT

A power source device for electrical discharge machining, in which a large machining current in excess of a certain present limit is automatically reduced while continuing the machining operation. A current-detecting resistor is provided in series with a discharge gap for machining, which resistor generates an output when the machining current becomes too large. A switching element responds to the output from the current-detecting resistor and reduces the machining current.

United States Patent Kondo POWER SOURCE DEVICE FOR 51 .Jan. 18,1972

OTHER PUBLICATIONS ELECTRICAL DISCHARGE Millman & Halkas, ElectronicDevices and Circuits" 1967, MACHINING P [72] Inventor: Iwao Kondo, 39-9Kita-machi l-chome, primary v h Tokyo, Japan Assistant Examiner-RobertO'Neill [22] Filed: Jan. 21, 1970 Attorney-Waters, Roditi, Schwartz andNissen 21 Appl. No.: 4,536 [57] ABSTRACT A power source device forelectrical discharge machining, in [30] Foreign Application PriorityData which a large machining current in excess of a certain presentlimit is automatically reduced while continuing the machining Jan. 21,1969 Japan ..44/4538 operation A cunenpdetecting resistor is provided inSeries [52] u 8 cl 219/69 C with a discharge gap for machining, whichresistor generates [51] 9/16 an output when the machining currentbecomes too large. A [58] i 219/69 switching element responds to theoutput from the current-de- Searc taming resistor and reduces themachining current. [561 References Cited 9 Claims, 18 Drawing FiguresUNITED STATES PATENTS 3,178,551 4/1965 Webb ..2l9/69 I I VI I 7' 1 2 4r' 7 H i L 1 4 PATENTED JAM 8:972 3.638.295

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PATENTED JAN 3 8 I972 SHEET 1101 11 FIGJ? FieJa SERVO VOLTAGE IN CURRENTTHROUGH THE DISCHARGE GAP POWER SOURCE DEVICE FOR ELECTRICAL DISCHARGEMACHINING This invention relates to a power source device for electricdischarge machining, which produces an electrical discharge currentbetween an electrode and a workpiece, so that the workpiece is partiallyremoved by the discharge current for effecting desired work thereon,such as perforation, shaping, engraving, cutting, grinding, and thelike.

An object of the present invention is to provide a power source devicefor electrical discharge machining, based on a series switching systemof the emitter follower type, which has heretofore been considered to beimpossible to construct.

Another object of the present invention is to provide an independentimpulse type power source, in which a short-circuit current is limitedto a level lower than the discharge current level for machining asopposed to a known power source of independent impulse type, whereinupon occurrence of a short circuit across a discharge gap, ashort-circuit current flows, whose magnitude is larger than that of thenormal discharge current for machining.

It is another object of the present invention to provide an improvedpower source device for electrical discharge machining, in which amaximum value (peak value) of the machining discharge current ispresent, depending on the magnitude of a machining area (that is asurface area of an electrode which faces a workpiece so as to define adischarge gap), in such a manner that the discharge current can beincreased up to the preset value, as the machining conditions vary. Ifthe machining operation becomes unstable, the machining dischargecurrent is immediately reduced.

Other objects and advantages of the present invention may be appreciatedby referring to the following description and claims, taken inconjunction with the accompanying drawings, in which:

FIGS. la, lb, and 1c are schematic diagrams collector-follower-type,transistor switching circuits of series switching system of thecoIIector-follower-type, parallel switching system, and series switchingsystem of the emitter-followertype, respectively;

FIG. 2 is a circuit diagram of a power source device, using a seriesswitching system of the collector-follower-type;

FIG. 3 is a circuit diagram of a power source device, based on theparallel switching system;

FIG. 4 is a diagrammatic illustration of a power source device, usingthe series switching system of the emitter-follower-type;

FIG. 5 is a simplified circuit diagram of a power source device,according to the present invention;

FIG. 6 is a circuit diagram of a power source device of the invention,including a high-voltage generating means;

FIG. 7 is a circuit diagram of the power source device of FIG. 6,including a means for controlling the machining maximum value of thedischarge current;

FIG. 8, 9, and 10 are circuit diagrams of different embodiments of theinvention, respectively;

FIG. 11 is a circuit diagram of a logical control circuit;

FIG. 12 is a graph, showing the relation between the magnitude of themachining discharge current and the voltage across dischargingelectrodes;

FIGS. 13a to 13d are graphs, illustrating the characteristics of voltageacross the discharge gap in the circuit of FIG. 6;

FIG. 14 is a graph, showing the characteristics of current for machiningin the circuit of FIG. 7;

- FIG. 15 is a graph, showing the'time variation of a current through abranch element;

FIGS. 16 and 17 are circuit diagrams, illustrating the manner in whichthe power source device of the present invention operates; and

FIG. 18 is a graph, similar to FIG. 12, illustrating the operativecharacteristics of the circuits of FIGS. 16 and 17.

Like parts are designated by like numerals and symbols throughout thedrawings.

Before describing the details of the present invention, three differenttransistor switching circuits, which may be used in the power sourcedevice of the invention, will be described, referring to FIGS. Ia to lc.The transistor switching circuits per se of FIGS. la to 1c are known tothose skilled in the art. in the following description it is assumedthat the power source impedance of the switching circuits of FIGS. la to1c is so small as to be negligible.

In FIG. la, a series switching system of the collector-follower typeuses a transistor chopper in a circuit for converting a direct currentinto a series of pulses by intermittently chopping. The transistorchopper of FIG. 1a comprises a switching element (transistor) 1 and aload impedance Z,,, which are connected in series with each other, whileconnecting the load impedance 2,, to the collector of the transistor 1.Thus, when the switching element 1 is turned on, an electric currentflows through the load impedance Z,,. Woolman and Growelt in theirJapanese Patent Publication No. 9,399} I966, disclosed a power sourcedevice for electric discharge machining, which is based on thecollector-follower type transistor chopper, as shown in FIG. la. Theload impedance 2,, in the circuit of FIG. 1a is a passive two-terminalimpedance, which consists of a discharge gap between electrodes, leadwires, a series resistor (a current-limiting resistor), a diode, etc.

A parallel switching system, as illustrated in FIG. lb, also constitutesa transistor chopper. In this circuit, a switching element(transistor 1) is connected in parallel with a load impedance Z,,, and acurrent-limiting impedance Z is connected between a power source 2 and ajoint of the aforesaid switching element and the load impedance.Accordingly, the magnitude of a short-circuit current in the circuit ofFIG. lb is determined by the current-limiting impedance and the voltageof the power source. The operation of the circuit of FIG. lb isdifferent from that of FIG. la in that an output current flows throughthe load impedance 2,, of FIG. lb only when the transistor 1 is turnedoff. Power source devices for electric discharge machining, based on theparallel switching system, as shown in FIG. lb, have been known,referring for instance, to US. Pat. No. 3,056,065, which was granted toCecil P. Porterfield on Sept. 25, 1962, and Japanese Patent PublicationsNo. 16,000/l967 and No. l5,040/l968, both granted to Toshio Asaeda andIwao Kondo. The current-limiting impedance Z of FIG. lb is atwo-terminal impedance including a resistance, an inductance, acapacitance, and a semiconductor element.

In the aforesaid two systems, the presence of the load impedance 2,, orthe current-limiting impedance Z, acts to protect the transistor 1 andthe power source 2 from detrimental effects of a short circuit at adischarge gap contained in the load impedance Z,,.

Actual power source devices for electrical discharge machining, based onthe known collector-follower type series switching system and the knownparallel switching system will now be described, together with theadvantages and disadvantages thereof.

Both a dependent impulse circuit (in which the discharging perioddepends on the physical conditions of discharge gap between electrodes)and an independent impulse circuit (in which the discharging period isindependent of the physical conditions of the discharge gap betweenelectrodes) are used as an impulse generator for electrical dischargemachining. FIG. 5 illustrates an example of the independent impulsecircuits. In the figure, a pulse generator 23 intermittently actuatesswitching elements 1 and 39. The output wave from the pulse generator 23may he of any suitable shape, and any suitable duty factor may beassigned to the pulse oscillator.

FIG. 2 shows an actual circuit for a power source device for electricaldischarge machining, based on the series switching system of thecollector-follower type. The circuit of FIG. 2, in order to cause adischarge across an electrode 3 and a workpiece 4 requires the breakingof the insulation therebetween. At the same time, the magnitude of thedischarge gap 5 must be maintained at a value suitable for the desiredmachining operation. Therefore, the voltage of the power source 2 shouldbe comparatively high, e.g., 50 to 100 volts. The arc voltage during thedischarge across the electrode 3 and the workpiece 4 is less than 30volts, and it is considerably lower than the output voltage of the powersource 2. The difference between the arc drop and the output voltage ofthe power source 2 is absorbed by the voltage drop across acurrentlimiting resistor 6.

Transistors I, I, and 1" are usually connected in parallel. When thetransistor switch, made of the last-mentioned three transistors, isturned on, the emitter-collector voltage is less than 1 volt. Resistors7, 7, and 7", each being of 0.2 to ID ohms, are connected to theemitters of the aforesaid transistors, with the intent of equalizing theelectric currents flowing through the transistors l, l, and I". Aresistor 8 is used for turning off the transistors completely when thereis no output from the pulse generator 23.

If the electrode 3 is short circuited with the workpiece 4, in thedevice of FIG. 2, the short circuit current I is approximatelyequivalent to the quotient of the resistance R of the current limitingresistor 6 to the voltage V of the power source 2; namely, I =V/R. Suchrelation is shown in FIG. 12 as a straight line characteristics S.One-of the important features of the collector-follower-type seriesswitching system is in that the input power to the base of thetransistor 1 for intermittent interruption for electrical dischargemachining may be small. On the other hand, the collector-follower-typeseries switching system has the following shortcomings.

l. A large amount of heat is inevitably generated in the resistor 6.

2. The use of a Darlington circuit for the transistor 1 is verydifficult, because the maximum voltage applicable to the first-stagetransistor thereof is low (generally speaking, the maximum voltageapplicable across the collector and the emitter of a transistor withsmall capacity is low). As a result, a power source with a largecurrent-carrying capacity has to be used in the first stage circuit.

3. The short-circuit current is always larger than the machiningcurrent.

4. The transistor I must have a high withstanding voltage.

5. Either one of the electrode 3 and the workpiece 4 is not at the samepotential with that of the ground (emitter) of a driving circuit.

FIG. 3 illustrates an example of practical parallel switching systemsfor electrical discharge machining. With the circuit of FIG. 3, when thetransistors I, I, and I" are turned on, the

voltage across an electrode 3 and a workpiece 4 is reduced close to zero(for instance, less than about 1 volt), and the are through a dischargegap 5 is extinguished or interrupted. On the other hand, when thetransistors I, 1, and 1" are turned off, the voltage V of the powersource 2 is substantially. directly applied across the electrode 3 andthe workpiece 4, so as to initiate the discharge therethrough.Accordingly, the voltage V of the power source 2 in this parallelswitching system is substantially equivalent to that of the seriesswitching system. Under certain conditions, by inserting an inductance 9in series with a current-limiting resistor 6, as shown through dottedlines in FIG. 3, the voltage of the power source 2 can be selected to besomewhat lower than the aforesaid value.

The advantages of the parallel switching system are as follows:

1. Either one of the electrode 3 and the workpiece 4 can be 7 2. Thetransistors I, I, and I" must have a very high withstanding voltage.

3. The short-circuit current, in response to the short circuit of theelectrode 3 with the workpiece 4, is larger than the machining current,as in the case of the aforesaid series switching system.

4. The use of the Darlington circuit is almost impossible in theparallel switching system, in view of the same reasons as those of thecollector-follower type series switching system.

Referring to FIG. Ic, there is provided an emitter-follower type seriesswitching system. One of the important features of the emitter-followertype switching system is in the smallness of its output impedance. Onthe other hand, if the discharge gap (or the load impedance Z,,) isshort circuited, there will be no current-limiting impedance in a closedcircuit. Accordingly, and excessively large current flows through atransistor I, which may in the worst instance, damage the transistor 1.Due to such shortcoming, the emitter-follower type transistor chipperhas not been heretofore used as a power source circuit for theelectrical discharge machining operation.

An object of the present invention is to provide a means for detectionof an increase of machining current in excess of maximum value, so as toreduce the machining current, whereby the efficiency of electricaldischarge machining operation can be improved.

In order to limit the maximum machining current, it has been proposed touse a plurality of switching units 1,, each consisting of switchingelements like I, l, and 1", connected in parallel, and to connect theparallel switching unit 1,, in series to the machining electrode ofcollector-follower type switching system, so that the switch units maybe operated in parallel under normal machining operation, while turningoff all the switching units except one upon increase of the machiningcurrent in excess of a certain predetermined value. Whereby, the overallimpedance of the circuit increases, so as to reduce the machiningcurrent, as shown by Curve 1 in FIG. 12.

However, the use of such parallel switching units inevitably makes theoverall circuit complicated.

Therefore, an object of the present invention is to provide acurrent-limiting circuit of simple construction, by connecting adischarge electrode and a current-detecting resistor in series with eachother, and then connecting a voltage divider in parallel to theseries-connected electrode and the current-detecting resistor. Theincrease of the machining current in excess of a certain predeterminedvalue is detected by the current-detecting resistor. Thecurrent-detecting resistor generates an output signal upon detection ofsuch an excessively large machining current, and with the output signalcontrolling bypass element. The bypass element in turn controls theswitching element so as to reduce the machining current.

Robert S. Webb US. Pat. No. 3,178,551, granted on Apr. 13, I965,discloses a current-sensitive per-pulse cutoff circuit, which includes acurrent-detecting element and is used for interruption of machiningcurrent in an electrical discharge machining device. Thecurrent-detecting element in Webb's circuit, however, is inserted inseries between the collector of a transistor and a power source. Inother words, the switching transistor of Webbs circuit operates as acollector-follower type element. Accordingly, a transistor for currentdetection in the Webbs circuit must have a high withstanding voltage.Furthermore, in order to cut off a transistor switch in Webb's circuit,at least one phase-converting or phase-inverting transistor must beprovided immediately after the current-detecting transistor.

Webbs circuit has a further limitation in that the output voltage from apower source, e.g., storage batteries, is applied to a discharge gapacross an electrode and a workpiece, through a resistor, which isintended both for current detection and current limitation. In otherwords, the machining voltage is also used for insulation breakdown.Thus, with Webb's circuit, the power source voltage must be higher thanas in the present invention. More particularly, the operativecharacteristics of Webbs circuit are similar to those, as represented bystraight lines U, U of FIG. 12. The capacity of the power source inWebb's circuit must be about twice as large as that of the presentinvention. With the circuit of the present invention, theefficiency ofthe power is doubled as compared with that of Webb's circuit, inperforming the same function. The use of a synchronized high voltage inthe present invention has made such an improvement in efficiencypossible.

The invention will now be described in further detail, by referring toan emitter-follower type transistor switching system, which isparticularly suitable for fulfilling the object of the invention. FIG. 4illustrates a power source device for electrical discharge machining,based on an emitter follower transistor switching circuit. The voltage Vof the power source of the circuit of FIG. 4 should be the same as thatof the circuit of FIG. 3, because in both circuits, the insulation ofthe discharge gap 5 must be broken through while maintaining a certainmagnitude thereof.

A current-limiting resistor 6 must be connected, in series, to the gap5. If no such current-limiting resistor is provided, upon eventualestablishing of a short circuit between the electrode 3 and theworkpiece 4, such an excessively large electric current which flowsthrough the transistors 1, 1', and 1" may instantaneously damage thetransistors. Such conditions are represented by a straight line P inFIG. 12.

As a feature of an emitter-follower type circuit, the input voltagesbetween the common base terminal B of the transistors 1, 1', and 1" andthe ground terminal E should be as high as the full power source voltageV.

Accordingly, the circuit of FIG. 4 cannot be applied directly to anactual power source device for electrical discharge machining, even if acurrent-limiting resistor 6 is inserted in series with the dischargegap.

In a preferred embodiment of the present invention, as shown in FIG. 5,a current-detecting resistor 10, an electrode 3, and a workpiece 4 areconnected in series to a transistor switching element means 1,. Avoltage divider consisting of resistors l1 and 12 is connected inparallel to the aforesaid se-, ries circuit including thecurrent-detecting resistor 10, the electrode 3, and the workpiece 4. Oneend of the voltage divider is connected to the emitter of the switchingelement means I A bypass transistor 14 has a base connected to theresistor 11 of the voltage divider, a collector connected to the base ofthe switching element means 1,, and an emitter connected to a juncturebetween the current-detecting resistor 10 and the machining electrode 3.A resistor 13 is connected, in series, to the base of the switchingelement means 1 so as to feed an electric current to the base from thecollector circuit.

The switching element means 1,, of FIG. 5 comprises three transistors I,l, and 1", which are connected in parallel with each other, and thebases of the three transistors are joined together, so that the junctureis connected to the collector circuit of a bypass transistor 14. Thecurrent-detecting resistor senses the magnitude of the machining currentand the presence of a short-circuit current, and the resistance value ofthe resistor 10 is smaller than that of the current-limiting resistor 6in the circuit of FIG. 4. The resistance value of the cur rent-detectingresistor 10 is approximately given by wherein I represents ashort-circuit current (See FIG. 12).

As the insulation of the discharge gap 5 defined between the machiningelectrode 3 and the workpiece 4 collapses, a machining current flowsthrough the discharge gap, as shown by an arrow 15. As a result, thereis produced a voltage drop across the current-detecting resistor 10,with a potential at the terminal F being positive relative to that ofthe terminal H. This voltage drop falls in a range of 2 to 3 volts tothe utmost for normal machining currents. A fraction of the outputvoltage (i.e., the voltage across the discharge gap) is given betweenthe terminals F and J, by means of the voltage divider includingtheresistors 11 and 12. The resistor 11 has a slidable terminal J, which isso adjusted that, during the normal machining operation, the voltagedrop across the terminals F and J is the same as the voltage drop acrossthe current-detecting resistor 10. In other words, the voltage dropacross the terminal J and H is zero for normal machining operation.

Accordingly, there is, as a result, no collector current in the bypasstransistor 14. Referring to FIG. 12, as the machining current exceeds apreset value I the voltage drop across the terminals F and H surpassesthat between the terminals F and J, so that the terminal J becomespositive as compared with the terminal H. Consequently, the polarity ofthe voltage between the terminals J and H is changed, in response to theaforesaid increase in the machining current.

In view of the foregoing, a collector current flows through the bypasstransistor 14, as shown by an arrow 16 (FIG. 5). In other words, acurrent, which previously flowed into the bases of the transistors I, land 1" through the resistor 13, is now bypassed through the transistor14 in the direction of the arrow 16. Thus, the exciting base current ofthe transistors 1, l, and 1" is reduced, so as to reduce the machiningcurrent, as shown by a line Q in FIG. 12. At the same time, the outputvoltage is also reduced.

Succinctly, with the circuit of FIG. 5, when the machining electrode 3is short circuited with the workpiece 4, the machining current isreduced. Thus, the transistors 1, 1, and 1 are never damaged throughsuch a short circuit.

The magnitude of the short-circuit current in the circuit of FIG. 5depends almost solely on the resistance value of the current-detectingresistor 10. The short-circuit current is usually about one-third of themaximum machining current.

In order to further limit the short-circuit current (e.g., to aboutone-tenth of the maximum machining current), the bypass transistor 14,which acts as a simple amplifier may be replaced by a differentialamplifier 27, as shown in FIG. 8. The use of the differential amplifier,however, necessitates an auxiliary power source 28.

In lieu of the circuit of FIG. 5, it is possible to use the circuit ofFIG. 9. The circuit of FIG. 9 is also an emitter-follower typetransistor switching circuit, similar to that of the circuitof FIG. 5.The relative position of the current-detecting resistor 10 and thedischarge gap 5 in the circuit of FIG. 9 is contrary to that of thecircuit of FIG. 5. In essence, one of the differences between circuitsof FIGS. 5 and 9 lies in the position of the currentdetecting resistor10 in the overall circuit.

FIG. 10 illustrates another circuit, which is essentially acollector-follower circuit. The operative characteristics of thecollector-follower-type circuit of FIG. 10 can be made similar to thoseof an emitter-follower-type circuit, by directly connecting thecollectors of a switching element means 1,, to a power source, withoutpassing through a current'limiting resister, and by not using acurrent-limiting resistor in the emitter circuit. The circuit of FIG. 10is different from those of FIGS. 5, 8 and 9 in that separate powersources 17 and 18 are provided, and that a terminal H (connected to amachining electrode 3) is used as a reference (group) point during themachining operation.

The difference between the aforesaid emitter-follower type circuit andthe collector follower type circuit can be said to lie in as to whetherthe input signal is applied across the base and the ground through aload resistor, or is directly applied across the emitter and the base.Since the internal resistance of the power source is assumed to benegligible, it is not critical as to whether to connect a load (thedischarge gap and the currentlimiting resistor) to a collector circuitto an emitter circuit.

With the aforesaid circuit arrangement, the following features can thusbe achieved.

1. It has been heretofore considered to be almost impossible to use anemitter-follower type transistor switching circuit in an electricaldischarge machining device. The inventor has succeeded in overcomingsuch difficulty by incorporating a short-circuit-protecting circuit inthe emitterfollower-typc transistor switching circuit.

2. By combining a circuit for limiting the machining current and anemitter-follower type transistor chopper, the output voltage of a powersource 2 can be reduced from 40 to 20 volts. At the same time, acurrent-limiting resistor 6 may be eliminated. As a result, heat loss inthe resistor can be mitigated. Consequently, the power efficiency cangreatly be improved by the present invention.

In a commonly used power source device for independent impulse typeelectrical machining, the short circuit current is always larger thanthe machining current during the machining operation. In the circuit ofthe invention, however, the short circuit current I S is reduced toabout one-third of the machining current and such a short circuitcurrent does not give any detrimental effects to the power source 2 andthe transistors l, 1 and 1".

4. Resistors having small resistance values are disposed in the circuitof the invention, however such resistors, namely 7, 7', 7", and 10, donot limit the machining current by themselves. The resistor 10 isintended for current detection, whereas the resistors 7, 7' and 7" arefor equalizing the currents through the transistors l, 1 and 1".

. The use of an emitter-follower-type transistor chopper makes itpossible to directly connect the ground wire of a pulse generator 23 toa workpiece 5 (forming one of the electrodes of a discharge gap). Withthe direct connection or common junction of the workpiece and the pulsegenerator, the construction of other circuit elements is simplified andtheir operation is stabilized; namely, a circuit for periodicallyraising the electrode, a servo circuit, a circuit for automaticallymeasuring the conductance of the discharge gap, a circuit forautomatically measuring the working area, etc.

6. The machining current of various magnitudes may be regulated by usinga suitable number of transistor switching circuit units 24 in parallel.

7. An emitter-follower-type transistor chopper is characterized in thatits output impedance is very small as compared with that of otherchoppers of similar type. Accordingly, the regulation of the outputvoltage is considerably improved, and the power efficiency iscorrespondingly improved.

As can be seen from the lines Q and Q in FIG. 12, with theemitter-follower-type transistor switching circuit, the output voltagevariation is very small as long as a nonnal machining current I flowstherethrough. If a power source voltage is high, a current-limitingresistor 6 must be inserted in the passage of the machining current I(along the arrow of FIG. 5), in series therewith, so as to compensatefor the dif- 'ference between the power source voltage and the dischargearc voltage (about 30 to 15 volts) at the normal machining current I Ifsuch current-limiting resistor 6 is used, the heat generation in thecircuit is increased, and the power efficiency deteriorates. (Theperformance characteristics become those shown by lines U, U in FIG. 12,and the power efficiency is lowered down to about 50 percent.)

On the other hand, if the output voltage of the power source 2 isselected to be only slightly higher than the discharge arc voltage(about 30 to 15 volts), the power consumption at the discharge gapduring the machining amounts to 80 percent or more of the output powerfrom the power source 2. (In effect, the power efficiency is improved upto 80 percent or higher.) In FIG. 5, the low output voltage of the powersource 2 allows the use of transistors l, l, and 1" with a comparativelylow withstanding voltage. At the same time the Darlington circuit may beused in the exiting stage (or a prestage) of the transistors I, l, andI" (see FIG. 7). As a result, the performance characteristics becomethose shown by lines W, W in FIG. 12.

The use of a low output voltage V of the power source 2, however, tendsto provide a too low voltage across the discharge gap 5, which may causesome difficulties in machining operation. In order to mitigate suchdifficulty, it is proposed to use a switching unit 25 for high-voltagegeneration, as shown in FIG. 6. The output from the switching unit 25 isstepped up by a transformer 19, and then rectified by a rectifier 20, soas to generate impulses, as shown by the wave form a in FIG. 13a. Thepulses from the switching unit 25 are superposed on the output from thetransistor switching unit 24, having its waveform represented by thesymbol B of FIG. 13b. The superposed voltages are represented by thewaveform y of FIG. 13c, and the combined voltage is then applied acrossthe discharge gap 5 between the electrode 3 and the workpiece 4.

In the circuits of FIGS. 6, 7 and 8, the output from the switching unit25, which is similar to the transistor switching unit 24 inconstruction, is stepped up by the transformer 19. The high voltage thusgenerated is then rectified by a rectifier 20 and applied across theelectrode 3 and the workpiece 4. The inventor discloses the machiningwith low-voltage highcurrent pulses in his Japanese Utility ModelApplication No. 57,996/1964, Japanese Patent Application No. 95,224/I968 and Japanese Patent Application No. 60,964/1964, in which ahigh-voltage blocking diode 43 (FIG. 16) is connected in series to a DCswitching circuit, so that the output from the switching circuit isstepped up by a transformer and then rectified, for producing a highvoltage for breaking through the insulation of the discharge gap. Afterthe insulation breakthrough, the machining is then carried out bylow-voltage high-current pulses.

More particularly, the insulation of the discharge gap 5 collapsesthrough the aforesaid impulse high voltage V,, (FIG. 12). Accordingly,the fluctuation in the magnitude of the discharge gap 5 may be regulatedby varying or switching the output voltage from the transformer 19.After the collapse of the insulation through the discharge gap 5, thevoltage applied thereto is reduced, as shown by the waveform '0 of FIG.13c, while large current pulses flow therethrough, as shown by thewaveforms e of FIG. 13d.

In FIG. 6, resistors 21 and 22 constitute a voltage divider forpreventing the generation of an abnormally high voltage.'The resistor 21also acts as a current-limiting resistor in the highvoltage generatingcircuitry, in the event of discharge or short circuit across theelectrode 3 and the workpiece 4.

It is well known in electrical discharge machining that the magnitude ofthe machining current depends on dimension of the electrode surface(hereinafter referred to as the machining area,), which consists of anelectrode and a workpiece disposed in face-to-face relationship defininga discharge gap. In other words, for processing a larger machining area,a larger value is selected for the machining current, while converselyfor processing a smaller machining area, the machining current selectedis smaller. However, the direct measurement of the machining area duringthe machining operation is almost impossible to attain. In practice, ithas been tried to determine the machining area and the machining currentbased on the following criteria.

I. The normally available maximum machining current is determined, basedon the approximate value of the machining area, which is presumed fromthe shape and dimension of the electrodes.

2. The machining operation is started with a machining current slightlysmaller than that as determined by the preceding item (I).

3. It is estimated whether the initial machining current is too large ortoo small, based on the sound and the operation of the servos, which areaffected by the number of repetitions of short circuits (or repetitionfrequency of discharge) per unit time at the discharge gap between theelectrode and the workpiece.

4. The indication of an ammeter for the machining current is read forthe purpose of reference.

5. The steps (3') and (4), or the steps (1 (2), (3), and (4),

are repeated at uniform time intervals.

In the above itemized criteria, only the item (I) has a direct relationwith the dimension of the electrode or the machining area. Accordingly,the machining area may usefully be measured during the machiningoperation, by measuring, for instance, the capacitance between theelectrodes. Nevertheless, the circuits for such measurement will becomevery complicated. Furthermore, it is very difiicult to maintain constantmeasuring conditions (e.g., keeping the discharge gap constant).

It is one of the primary objects of the present invention that a stablemachining current is automatically established by using a specialimpulse-generating circuit. More particularly, according to the presentinvention, no direct measurement is made on the capacitance or themachining area. When the commonly usable maximum machining current, asdetermined by the item (I), is maintained at a set value (either anultimate value or a normal value), and when the machining currentincreases, in order to effect an increase of the repetition frequency ofthe short circuit per unit time, the magnitude of the machining currentis reduced in dependence upon the repetition frequency of the shortcircuit, by means of the special arrangement in the impulse generatingcircuit.

In the collector-follower-type series transistor switching system ofFIG. 2, or in the case of the transistor parallel switching circuit ofFIG. 3, the resistance value of the currentlimiting resistor 6 must bemodified in order to change the machining current. Accordingly, if it isdesired to change the machining current during the machining operation,depending upon the machining conditions, the resistance value of thecurrent-limiting resistor 6 must be made varied by using a large numberof relays, or alternatively a portion of the transistors 1, l, and 1" ofFIG. 2 must be temporarily turned off. With that type of arrangementhowever, much time is consumed and the construction of the circuitrybecomes complicated.

According to the present invention, in order to fulfill the aforesaidobject, a discharge electrode and a current-detecting resistor areconnected in series, and a voltage divider is connected in parallel withthe series circuit thus prepared. A branching or shunting element isprovided in the voltage divider, so that the maximum value (peak value)of the machining current can be determined by the adjustment of the rateof shunting by the shunting element.

FIG. 7 illustrates an embodiment of the present invention. A dischargeelectrode 3, a workpiece 4, and a current-detecting resistor 10 areconnected, in series, to a transistor switching element means 1 Avoltage divider, consisting of resistors 11 and 12, is connected inparallel to the series connected circuit including the current-detectingresistor 10, the electrode 3, and the workpiece 4. One end of thevoltage divider is connected to the juncture between the switchingelement means 1,, and the current-detecting resistor 10. A base currentshunting element 14 of the switching element means 1 which operates bycomparing the voltage drops across the currentdetecting resistor andacross the voltage divider resistors 11 and 12, is so connected as to beparallel both to the switching element means 1,, and to thecurrent-detecting resistor 10. Another current-shunting element 26 isconnected in parallel to the resistor 12 of the voltage divider. Therate of current-shunting is varied, depending on the conditions of thedischarge gap 5, so that the machining current can be increased to apreset maximum value.

As a means for varying the rate of current-shunting in response to theconditions of the discharge gap 5, a voltage detector 29 is provided inparallel to the discharge gap 5, while a machining current detector 30is provided in the circuit of the electrode 3 and the workpiece 4,whereby the output signals from the detectors 29 and 30 are fed to alogical control circuit 31. The output from the logical control circuit31 is used for regulating the current-shunting element 26, so as tocontrol the rate of current-shunting.

FIG. 11 is a schematic diagram of an embodiment of such logical circuit31, which consists of a pair of Schmitt circuits. The output terminalsof the detectors 29 and 30 are connected to the input terminals of thetwo Schmitt circuits, so that a capacitor 32 is charged onlywhen boththe output voltage and the machining current are present, i.e., onlywhen both of the detectors and 30 are on. As the capacitor 32 ischarged, the output from the transistor 33 increases, so as to verygradually increase the rate of shunting hy the shunting transistorelement 26. As a result, the maximum value of the machining currentthrough the transistors l, l, and l" is increased.

On the other hand, if the discharge gap 5 is reduced to a state close toshort-circuit, the output voltage is almost completely diminished(detector 29 being off), while the machining current is maintained(detector 30 being on). Ac cordingly, the rate of shunting is rapidlydecreased, so as to reduce the magnitude (intensity) of the machiningcurrent comparatively quickly. Similarly, if the discharge gap 5 iscompletely insulated without any machining current, the output voltageis present (detector 29 being on), however, there is then no machiningcurrent (detector 30 being off), and the logical circuit acts to reducethe rate of shunting by the shunting element 26. The short circuitcurrent i acts independent of the rate of shunting, as shown in FIG. 14.

The illustrated embodiment is actuated by the mean value of the pulsesnamely by a DC voltage. It is possible to use a circuit without thecharging capacitor 32, whereby the circuit can be actuated directly bythe pulses. In the circuit of FIG. 7, when the two detectors 29 and 30are on simultaneously, the output from the logical circuit 31 increasesthe base current of the shunting element 26, so that the collectorcurrent (as shown by the arrow 34) increases, thereby intensifying thevoltage drop across the resistor 11. FIG. 15 shows the variation of thecollector current as a function of elapsed time. As shown in the FIG., astable discharge is maintained through the discharge gap, with shortcircuits occurring from time to time, while the machining current(machining speed) is maintained substantially at a constant level. (seeFIGS. 13a to 13d). Since the collector current of the shuntingtransistor element 26 is limited to below a certain value by means of aresistor 35, the machining current (machining speed) does not increasein excess of a predetermined level.

If a short circuit between the electrode 3 and the workpiece 4 ismaintained for more than a predetermined period of time, the logicalcontrol circuit 31 is actuated, so as to reduce the maximum machiningcurrent. As a result, if the resistance value of the resistor 35 whichis in series with the shunting transistor element 26 is made variable inresponse to the maximum machining current, the maximum value of themachining current through the machining circuit can be changed,depending on the machining area, as shown by I l,, ,...l,,,' in FIG. 14.

In order to obtain the aforesaid increase of the machining current, thetime constant of the collector current circuit 34 of the shuntingtransistor element 26 should preferably fall in the range of about 0.1sec. to about l0 sec. for most machining applications.

In order to obtain the reduction of the machining current, thecorresponding time constant of the collector current circuit shouldpreferably fall in the range of about 0.01 sec. to about 1 sec.

The distinctions of the circuit of the present invention over that ofWebb's US. Pat. No. 3,178,551 will now be described. In the circuit ofthe invention, the current-detecting resistor 10, the electrode 3, andthe workpiece 4 are disposed on the emitter side of the switchingtransistors l, 1', and 1". Accordingly, the circuit on the side of theworkpiece 4 may be grounded, while such grounding can not be made inWebb's circuit. The grounding system of the circuit of the presentinvention is therefore preferable for the circuit control.

The transistor l4 of the circuit of the invention corresponds to atransistor 114 of Webb's circuit. The insulating strength of thetransistor 14 can be lower than that of the transistor 1 14.

The collector of the current-controlling transistor 14 of the presentinvention is either directly connected to the bases of the switchingtransistors l, 1', and l", or directly connected to the base of acurrent-amplifying transistor 40, which constitutes a Darlington circuittogether with the switching transistors l, 1', and 1". As a result,according to the present invention, a linearity is unconditionallyensured in a loop, which consists of the base input of thecurrent-controlling transistor 14, collector of the transistor 14, thebase of the transistor 40, the bases of the switching transistors l, l,1", the emitters of the transistors l, I, l, emitter resistors 7, 7 7",current-detecting resistor 10, and a voltage across the terminals .l andII depending on the voltage drop across the variable resistor 11.Accordingly, with the circuit of the present invention, the electricaldischarge machining be stably effected in a current reducing zone alongthe lines U and W in FIG. 12. In essence, with the circuit of thepresent invention, there is an elimination of rapid jumps between theoperating point M and the short-circuit current I Furthennore, if onlythe maximum machining current value is set at the tenninal J of thevariable resistor 11, the characteristics curves U and W becomesubstantially rectilinear without any adjustment. Accordingly, theelectrical discharge machining and the automatic control of theelectrodes are possible within the range of the rectilinearcharacteristics U and W.

On the other hand, in the case of Webbs circuit, when the transistor 114is turned on upon detection of the maximum current, complementaryamplification is done in several steps by transistors 106, 98, and 84.As a result, the region from the operating point M to the short-circuitcurrent I is precipitously and sharply cut off.

In short, the construction of the detecting circuit per se is similar toWebbs circuit, however the characteristics of the circuits are vastlydifferent, due to the construction of the other portions of thecircuits.

FIG. 16 illustrates another embodiment of the present invention. In theFigure, and electrode 3, a workpiece 4, and resistors 11 and 12 areconnected to the emitter side of switching transistors l, 1', and 1",unlike Webbs circuit. When the machining current amounts to severalhundred amperes, the power loss in the current-detecting resistorbecomes very large. Accordingly, in the circuit of FIG. 16, a machiningcurrent-detecting resistor 10 is inserted between the switchingtransistor 1 andthe emitter of a current-amplifying transistor 40, whichconstitutes a part of a Darlington Circuit. The emitter of thetransistor switching element means 1,, is directly connected to theelectrode 3 through the currentdividing resistors 7, 7', and 7". One ofthe important features of the circuit of FIG. 16 is in that the powercapacity of the current-detecting resistor 10 becomes the inverse of thecurrent amplification factor of the transistors 1, l, and 1" (forinstance about l/20 to H50), and that almost 80 percent to 90 percent ofthe output from a power source 2 is consumed in the machining process.The electrode servomechanism is operated in response to the linearvoltage variation in the region between the operating point M and theshort-circuit current I (see FIG. 18).

FIG. 17 illustrates still another embodiment of the invention. In theFigure, a high-frequency constant voltage diode 44 is connected across avariable resistor 11 in a voltage divider. With the construction of thecircuit of FIG. 17, if the voltage across the terminals F and Kdiminishes, in response to a large machining current through theresistor 10, the voltage across the resistor 11, i.e., between theterminals F and L, may be retained at a Zenor voltage of the constantvoltage diode 44. Accordingly, the voltage-current characteristicsbetween the electrode 3 and the workpiece 4 produces a constant currentregion X, as shown in FIG. 18. The short-circuit current I,- of thecircuit of FIG. 17 is the same as that of FIG. 6. There is producedanother linear portion Y between the operating point N and theshort-circuit current I without any sudden change or jumps, as shown inFIG. 18. When applying the circuit of FIG. 17, the voltage variation inthe constant current region MN can be also used for automatic control ofelectrodes. Furthermore, the current carrying capacity of the powersource need not be larger than I Thus, the circuit can be constructed ata low cost.

The salient features of the present invention can accordingly besummarizedas follows.

I. The maximum machining current automatically reduces at the beginningand at the end of the machining operation. (With known devices, theoperators effected such control of the machining current, having to relyon experience.)

2. Upon occurrence of a short circuit between the electrode 3 and theworkpiece 4, the machining current becomes smaller than that for stablemachining operation. The magnitude of this short-circuit current issubstantially independent of the maximum machining current.

3. Only one control of the machining current, which is to be made by theoperation, is the setting of the maximum machining current. Thus,manpower is greatly saved.

4. By the introduction of a logical control circuit 31, the

aforesaid current and voltage control is made possible.

5. The resistance values of the resistors 11 and I2, constituting avoltage divider, can be considerably large, (for instance, several KOhms), and the current flowing through such resistors is very small.With a few ma. of the collector current 34 of the shunting transistorelement 26, the machining current can be controlled from several ten A.In other words, a large machining current is controlled by regulating asmall control current.

When a separate high-voltage generating circuit is used, as shown inFIGS. 6 to 8, the pulse generator 23 may be used for driving the twoswitching units 24 and 25. With such common pulse generator 23, the twoswitching units are operated in synchronism with each other, and thehigh voltage for collapsing the insulation of the discharge gap 5 isapplied thereto only when the switching unit 24 is on. A resistor 22,which is connected in parallel to the discharge gap 5 for preventingabnormal voltages, can be substituted by a constant voltage diode means37, as shown in FIGS. 7 and 8. Whereby, the high voltage for insulationbreakthrough, which is applied across the discharge gap 5, will becomesubstantially constant through the diode means 37. As the insulationbreakthrough voltage is stabilized by the diode 37, the magnitude of thedischarge gap 5 can also be kept substantially constant, for practicalpurposes.

In the examples of FIGS. 6 to 8, a transformer 19 is used for generatinga high voltage to be applied across the discharge gap for insulationbreakthrough. Any other suitable high-voltage generating means can beused, instead of the transformer 19, namely, a vacuum tube, a suitablepulser, an inductor generating a high counter voltage, and the like.

In the circuit of FIG. 6, a resistor 38 and a separate power source inseries therewith are provided for preventing a leakage current frombeing applied to the output terminals (leading to the electrode 3 andthe workpiece 4). Such leakage current tends to be generated when thetransistor 39 is turned on, while turning off the transistors 40, I, l',and I".

An auxiliary power source 41 in FIGS. 7 and 8 is useful in saturatingthe bases of the transistors 40, I, l and 1", when the transistor 39 isturned off while turning on the transistors 40, I, l', and 1". In orderto provide a large power source for supplying a large machining current,a suitable number of transistor switching units 24, may be incorporatedin the circuit in parallel with each other.

Instead of the series circuit consisting of the resistor 38 and theseparate power source in FIG. 6, an auxiliary power source 42 may beused, as shown in FIG. 8, for the purpose of preventing the propagationof the leakage current. In the circuit of FIG. 8, when the transistor 39is turned on, a voltage corresponding to the collector-emitter voltageof the transistor 39 (less than about 1 volt) is applied to thetransistors 48, l, 1, and 1" as a bias voltage. for completely turningoff the latter transistors.

In the circuit of FIG. 5, the addition of the current limiting resistors6, 6', and 6" will not affect the operative charac teristics of thecircuit, except that the slope of the operative characteristics line Wof FIG. 12 is changed. The power efficiency is, of course, reduced bythe addition of such resistors, but the increase in the slope of thelinear operative characteristics makes it easy to control theelectrodes.

in the circuit of the present invention, Darlington circuits are usedfor exciting the output stage transistors 1, 1', and 1". In other words,the collector of the transistor 40 is directly connected to thecollectors of the transistors l, 1', and 1". Accordingly, thetransistors l, l, and l" are not saturated during the machiningoperation. With such an arrangement, the linearity of the circuit isensured, and the linear operative characteristics U and W of FIG. 12 areclosely followed. The machining current gradually decreases whensurpassing the preset maximum value, but never be suddenly cut off. Whenthe circuit of the present invention is operated along the operativecharacteristics lines U and W, the transistors l, l, and l may be heatedto a certain extent, but such heating is not harmful at all forpractical purposes. The resistor 11 is illustrated as a variableresistor, but it may be constituted of a pair of accurately calibratedfixed resistors. The switching transistor element means 1,, is shown asa combination of three transistors in FIGS. 3, 4, 5, 6, 7, 8, 9, 10, 16,and 17. The number of transistors in the switching element means is notlimited to three, but any suitable number of transistors may be usedtherein. Furthermore, the pulse generator 23 of FIG. 8 may be such thatit operates in response to the voltage across the discharge gap.

Although the present invention has been described with a certain degreeof particularity, it is understood that the present disclosure has beenmade only by way of example and that various modifications in thedetails of construction and the combination and arrangement of parts maybe resorted to without departing from the spirit and the scope of theinvention as hereinafter claimed.

What is claimed is:

l. A power source device for machining a workpiece throughelectronically generating a discharge across a gap formed between saidworkpiece and an electrode, comprising an electric source, a transistorswitching element means, a current detecting resistor, a variablevoltage divider means and a bypassing transistor means, said switchingmeans and said resistor being arranged between said source and saiddischarge gap so that the collector circuit of said switching means isconnected with one terminal of said source while the emitter circuitthereof is connected through said resistor and said discharge gap to theother terminal of said source to form a series circuit, said voltagedivider means being connected in parallel to said series circuit so thatone end of said voltage divider means is connected to the emittercircuit of said switching circuit while the other end to a ground linefrom said gap to said other end of the electric source, said bypassingmeans being so arranged that the base and emitter circuits thereof areconnected respectively to a variably positioned contact to said voltagedivider means and to the series circuit between said resistor and saiddischarge gap while the collector circuit of said bypassing means isconnected to the base circuit of said switching means whereby when acurrent of the discharge is increased beyond a current value to be setby means of varying said voltage divider contact position as a desirablemaximum value then a signal detected across the junctions of the emitterand base circuits of said bypassing means in the form of polarityreverse actuates said bypassing means to control said switching meansfor decreasing the discharge current down to the preset maximum value.

' 2. A power source device as claimed in claim 1, in which saidswitching means consists of a plurality of transistors connected to oneanother with a collector, emitter and base circuit and cooperates with aprestige transistor of which the collector is connected with saidcollector circuit while the emitter of said prestige transistor isconnected to said base circuit so that the collector circuit of saidbypassing means is connected through the base of said prestigetransistor with the base circuit of said switching means.

3. A power source device as claimed in claim I, in which said bypassingmeans comprises a pair of transistors connected together with theircollector circuits which are connected to the base circuit of saidswitching means, a first of said bypassing transistor is connected atthe base and emitter thereof respectively to the variably positionedcontact of the voltage divider means and to the series circuit betweensaid resistor and said discharge gap, said power source device furthercomprising a pulse generator of which one terminal is connected to saidground line while the other terminal is connected to the base of thesecond bypassing transistor of which the emitter is connected to saidground line.

4. A power source device as claimed in claim 3, which further comprisesa high-voltage generating circuit adapted to be intermittently actuatedin synchronism with an ON and OFF of said transistor switching means sothat the output from said high-voltage generating circuit isadditionally applied across the discharge gap whereby said electricsource may be of lower voltage output.

5. A power source device as claimed in claim 1, in which said voltagedivider means comprises a variable resistor and a fixed resistor innerends of which are connected with each other so that the outer end ofsaid variable resistor is connected to the emitter circuit of saidswitching means while the outer end of said fixed resistor is connectedto said ground line, said power source device further comprising ashunting element for shunting a part of the current flowing through saidfixed resistor depending on a condition at the discharge gap whereby acurrent of the discharge may be increased.

6. A power source device as claimed in claim 5, in which said shuntingelement is a transistor of which the collector is connected to thevoltage divider between the variable resistor and the fixed resistor,the emitter of said shunting transistor being connected to said groundline, said power source device further comprising a voltage detectingmeans and a current detecting means arranged respectively for detectingthe discharge voltage and current of which outputs are fed into a logiccircuit means of which output terminals are connected respectively tothe base of said shunting transistor and to said earth line so as tocontrol said shunting transistor for automatically increasing thedischarge current depending on the discharge condition up to a presetmaximum current value.

7. A power source device as claimed in claim 6, in which said logiccircuit means comprises two Schmitt circuits connected with each other,a capacitor and an output transistor each input terminals of said twocircuits being fed with the respective output of said discharge voltageand current detecting means, so that only when both the voltage andcurrent detecting means are ON said capacitor is charged to increase theoutput of said output transistor whereby the collector current of saidshunting transistor is increased for increasing the shunted current.

8. A power source device as claimed in claim 2, in which said bypassingtransistor means comprises a differential ampli' fier consisting of twotransistors in pair and a third transistor, said pair of transistorsbeing connected together with their collectors and emitters, said powersource device further comprising a pulse generator and an auxiliaryelectric source so that the collectors of said differential amplifier isconnected to the prestige transistor base and the emitters of saiddifferential amplifier are connected to one end of said auxiliary sourceof which other end is connected to the series circuit between thecurrent detecting resistor and the discharge gap, the base of the firsttransistor of the differential amplifier being connected to the seriescircuit between the current detecting resistor and the discharge gapwhile the base of the second transistor is connected to said movablecontact of the variable resistor, said pulse generator being connectedat one terminal to said ground line while the other terminal isconnected to the third transistor base.

9. A power source device as claimed in claim 1, which further comprisesa constant voltage diode, such as a Zener diode, of which opposite endsare connected respectively at

1. A power source device for machining a workpiece throughelectronically generating a discharge across a gap formed between saidworkpiece and an electrode, comprising an electric source, a transistorswitching element means, a current detecting resistor, a variablevoltage divider means and a bypassing transistor means, said switchingmeans and said resistor being arranged between said source and saiddischarge gap so that the collector circuit of said switching means isconnected with one terminal of said source while the emitter circuitthereof is connected through said resistor and said discharge gap to theother terminal of said source to form a series circuit, said voltagedivider means being connected in parallel to said series circuit so thatone end of said voltage divider means is connected to the emittercircuit of said switching circuit while the other end to a ground linefrom said gap to said other end of the electric source, said bypassingmeans being so arranged that the base anD emitter circuits thereof areconnected respectively to a variably positioned contact to said voltagedivider means and to the series circuit between said resistor and saiddischarge gap while the collector circuit of said bypassing means isconnected to the base circuit of said switching means whereby when acurrent of the discharge is increased beyond a current value to be setby means of varying said voltage divider contact position as a desirablemaximum value then a signal detected across the junctions of the emitterand base circuits of said bypassing means in the form of polarityreverse actuates said bypassing means to control said switching meansfor decreasing the discharge current down to the preset maximum value.2. A power source device as claimed in claim 1, in which said switchingmeans consists of a plurality of transistors connected to one anotherwith a collector, emitter and base circuit and cooperates with aprestige transistor of which the collector is connected with saidcollector circuit while the emitter of said prestige transistor isconnected to said base circuit so that the collector circuit of saidbypassing means is connected through the base of said prestigetransistor with the base circuit of said switching means.
 3. A powersource device as claimed in claim 1, in which said bypassing meanscomprises a pair of transistors connected together with their collectorcircuits which are connected to the base circuit of said switchingmeans, a first of said bypassing transistor is connected at the base andemitter thereof respectively to the variably positioned contact of thevoltage divider means and to the series circuit between said resistorand said discharge gap, said power source device further comprising apulse generator of which one terminal is connected to said ground linewhile the other terminal is connected to the base of the secondbypassing transistor of which the emitter is connected to said groundline.
 4. A power source device as claimed in claim 3, which furthercomprises a high-voltage generating circuit adapted to be intermittentlyactuated in synchronism with an ON and OFF of said transistor switchingmeans so that the output from said high-voltage generating circuit isadditionally applied across the discharge gap whereby said electricsource may be of lower voltage output.
 5. A power source device asclaimed in claim 1, in which said voltage divider means comprises avariable resistor and a fixed resistor inner ends of which are connectedwith each other so that the outer end of said variable resistor isconnected to the emitter circuit of said switching means while the outerend of said fixed resistor is connected to said ground line, said powersource device further comprising a shunting element for shunting a partof the current flowing through said fixed resistor depending on acondition at the discharge gap whereby a current of the discharge may beincreased.
 6. A power source device as claimed in claim 5, in which saidshunting element is a transistor of which the collector is connected tothe voltage divider between the variable resistor and the fixedresistor, the emitter of said shunting transistor being connected tosaid ground line, said power source device further comprising a voltagedetecting means and a current detecting means arranged respectively fordetecting the discharge voltage and current of which outputs are fedinto a logic circuit means of which output terminals are connectedrespectively to the base of said shunting transistor and to said earthline so as to control said shunting transistor for automaticallyincreasing the discharge current depending on the discharge condition upto a preset maximum current value.
 7. A power source device as claimedin claim 6, in which said logic circuit means comprises two Schmittcircuits connected with each other, a capacitor and an output transistoreach input terminals of said two circuits being fed with the respectiveoutput of said discharge voLtage and current detecting means, so thatonly when both the voltage and current detecting means are ON saidcapacitor is charged to increase the output of said output transistorwhereby the collector current of said shunting transistor is increasedfor increasing the shunted current.
 8. A power source device as claimedin claim 2, in which said bypassing transistor means comprises adifferential amplifier consisting of two transistors in pair and a thirdtransistor, said pair of transistors being connected together with theircollectors and emitters, said power source device further comprising apulse generator and an auxiliary electric source so that the collectorsof said differential amplifier is connected to the prestige transistorbase and the emitters of said differential amplifier are connected toone end of said auxiliary source of which other end is connected to theseries circuit between the current detecting resistor and the dischargegap, the base of the first transistor of the differential amplifierbeing connected to the series circuit between the current detectingresistor and the discharge gap while the base of the second transistoris connected to said movable contact of the variable resistor, saidpulse generator being connected at one terminal to said ground linewhile the other terminal is connected to the third transistor base.
 9. Apower source device as claimed in claim 1, which further comprises aconstant voltage diode, such as a Zener diode, of which opposite endsare connected respectively at the opposite ends of a variable resistorof the voltage divider whereby discharge current characteristics arekept stable.